1. Field of the Invention
The present invention relates to the art of seismographic surveying, and more particularly, but not by way of limitation, to an improved hydraulic vibrator of the type used to generate continuous seismic signals.
2. Description of the Prior Art
Many seismic surveying systems have been developed which utilize continuous wave seismic signals generated in the earth by one or more vibrators. While a variety of vibrators have been developed and used heretofore, they generally comprise a base plate for engaging the surface of the earth, a reaction mass and a linear actuator, usually hydraulic, for reciprocating the reaction mass relative to the base plate at a desired frequency. A particularly satisfactory hydraulic vibrator which has heretofore been developed and used utilizes a fixed vertical double rod piston member and a reaction mass reciprocally disposed around the piston member forming a hydraulic cylinder. Pressurized hydraulic fluid is introduced to the cylinder to reciprocate the reaction mass by a conventional hydraulic fluid control system which alternately ports hydraulic fluid to the two sides of the piston.
While the sizes of the pistons and displacements of the hydraulic cylinders of such vibrators have heretofore been designed to achieve maximum vibration frequency range, such frequency range is often inadequate requiring the use of two or more vibrators of differing design. That is, in order to produce low frequency vibrations, a vibrator having a hydraulic cylinder of relatively large displacement and which moves relatively large volumes of hydraulic fluid in and out of the hydraulic cylinder is required. The same vibrator is usually incapable of achieving the high frequency vibrations required in some seismic surveys in that the displacement of the hydraulic cylinder is such that it is physically impossible to move the large volumes of hydraulic fluid in and out of the cylinder at such high frequency.
By the present invention an improved hydraulic vibrator is provided which includes apparatus for selectively changing the displacement of the hydraulic cylinder thereby allowing the vibrator to be operated over a broader frequency range.